Folks, I'm looking for the collective wisdom to trouble-shoot and solve this problem. The tach works, meaning the needle move when the engine is running, but when accelerating the needle moves slowly to the new RPM if at all.
The car in question is an 85, 300DS. I'm trying to put a trouble shooting process together, but I can find the schematic in the FSM to know what the components are that make up the tach system, nor do I know where it's sensors, or what sensors are used. Any help would be appreciated. Thanks

Greg

Circuit Diagram Index

Wiring Circuit: 1984-85 Model Year Page 120/1

Componet Locations:
Engine Speed Sensor: Page 212, Figure 2 - Under LH rear of engine at bellhousing

Top Dead Center Amplifier: Page 202, Figure 2 -in Diagnostic port on LH side wheel well (in the engine bay)

Usually culprit for intermittent tach operation is bad solder joints in the circuit board in the tach amplifier.

Sometimes, disassembling the tach amplifier and adding a cigarette filter between the housing and circuit board can restore functionality (albeit temporarily).

The cigarette filter acts like a shock absorber, preventing the circuit board from flexing too much.

The circuit board can be re-soldered, but you have to remove the anti-vibration elastomer encasing the componets.

Or you can just replace the tach amplifier with a good new or used one.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

the RPM amp that screws into socket on the fender,the pins,press the female sockets down.You need to find a shim that will keep them up,so the pins will make contact.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt

Thank you. This sounds like and electrical connection issue, at least. If I end up going the new tach amp route, do you have a part number and since this a essentially the same 617 engines used in the 300D's, could one be used from a 123 ?

I did a little more digging.

The MB part number 0015455932 is on the tach amplifier for both my 78 300SD and my 83 300SD.
But this part was only used until the end of the 83 model year.

Your 85 300SD doesn't have a tach amplifier in the diagnostic port.
The tach signal is routed through an early EGR computer (MB part number 0035455132). See attached post number 1 and thumbnail number 3 by Jeremy5848.
http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/226815-w123-parts-sale.html

The EGR computer receives its power from the OVP relay. See attached post number 3 also by Jeremy5848.
http://www.peachparts.com/shopforum/diesel-discussion/280208-tach-signal-wire-fuse.html

As Jeremy5848 suggests, for a failed or erratic tach in a 85 300SD, the likely culprit is either the OVP fuse, or the OVP relay itself.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022